In the fabrication process for semiconductor chips, a large variety of liquid chemicals are utilized in a large number of manufacturing steps required for processing an IC chip. For instance, during the fabrication of a semiconductor chip, a semi-conducting substrate such as a silicon wafer must be processed in a multiplicity of fabrication steps. These steps may amount to as high as several hundred processing steps which include etching, cleaning, deposition, or any other necessary steps for fabrication. During these manufacturing steps, a variety of chemicals including liquids and gases must be used to, for instance, etch a specific design feature on a silicon wafer, to clean after each etching or deposition steps, to deposit layers of dielectric or conductive materials on the substrate.
Since a large number of chemicals used in the semiconductor fabrication are toxic and/or corrosive, the handling or transportation of these chemicals must be dealt with extreme care and personal contact with these chemicals must be avoided as much as possible. Some of these chemicals are, for example, strong acids used in an etcher for forming a specific structure on a semiconductor wafer, or a photoresist liquid used in a photolithographic process for reproducing a pattern on a substrate. During most processing steps, the liquid chemical material is not completely consumed by the process and therefore, spent chemical liquids must be recycled or otherwise disposed.
One example that requires spent liquid chemical disposal in a semiconductor fabrication facility is the photoresist coating process. In a wafer fabrication plant, as many as 20 photoresist liquid coating machines are used to form photoresist layers for performing photolithographic processes. Each coating machine is equipped with a drain box for collecting spent photoresist liquid that was not coated on the wafer surface. These liquid drain boxes are monitored by a weight fencing device and are emptied manually when the drain box is full and an alarm is sounded to alert the machine operator to manually empty the drain box. This becomes a tedious and time consuming process which leads to a decrease in the machine yield. The conventional method of manually emptying the drain boxes therefore must be improved or eliminated.
It is therefore an object of the present invention to provide an automated processing liquid drain system that does not have the drawbacks or shortcomings of the conventional processing liquid drain tanks.
It is another object of the present invention to provide an automated processing liquid drain system that utilizes a buffer tank for accepting spent processing liquid from a machine drain box and then passing the liquid to a waste collection tank.
It is a further object of the present invention to provide an automated processing liquid drain system that utilizes a buffer tank for accepting spent processing liquid from a machine drain box which is equipped with a high liquid level sensor that prevents overflow in the buffer tank.
It is still another object of the present invention to provide an automated processing liquid drain system that utilizes a machine drain box, a buffer tank for accepting liquid from the drain box, and a multiple number of waste collection tanks for accepting liquid from the buffer tank and for switching over to an empty collection tank when the first collection tank is full.
It is still another object of the present invention to provide an automated processing liquid drain system that utilizes a machine drain box, a buffer tank and a multiple number of waste collection tanks controlled by a master controller which receives signals from liquid level sensors mounted on the buffer tanks and on the waste collection tanks.
It is yet another object of the present invention to provide an automated processing liquid drain system that utilizes a buffer tank for each process machine which is equipped with a high liquid level sensor and a low liquid level sensor for feeding signals to a master controller.
It is still another further object of the present invention to provide an automated process liquid drain system that utilizes logic circuits for controlling the switching between a multiple number of buffer tanks and a multiple number of waste collection tanks such that at least 16 process machines can be handled by the master controller.
It is yet another further object of the present invention to provide a method for draining spent processing liquid from a process machine by flowing a spent processing liquid from a machine drain box to a buffer tank equipped with level sensors and then to waste collection tanks monitored by liquid level sensors and a master controller for switching the spent process liquid to empty waste collection tanks upon the filling up of a first collection tank.